Paper is sheet material containing interconnected small, discrete fibers. The fibers are usually formed into a sheet on a fine screen from a dilute water suspension or slurry. Typically paper is made from cellulose fibers, although occasionally synthetic fibers are used. The wet strength of paper is defined (U.S. Pat. No. 5,585,456) as the resistance of the paper to rupture or disintegration when it is wetted with water. Paper products made from untreated cellulose fibers lose their strength rapidly when they become wet, i.e., they have very little wet strength. Wet strength of ordinary paper is only about 5% of its dry strength. Various methods of treating paper products have been employed to overcome this disadvantage.
Wet strength resins applied to paper are either of the “permanent” or “temporary” type, which are defined by how long the paper retains its wet strength after immersion in water. While wet strength retention is a desirable characteristic in packaging materials, it presents a disposal problem. Paper products having such characteristics are degradable only under undesirably severe conditions. While some resins are known to impart temporary wet strength and thus would be suitable for sanitary or disposable paper uses, they often suffer from one or more drawbacks. For example, their wet strength is generally of a low magnitude (about one-half of the level achievable for permanent-type resins), they are easily attacked by mold and slime, or they can only be prepared as dilute solutions.
There is a need, therefore, for improved methods for imparting appropriate levels of wet strength and/or repulpability to paper products.